Safety Issues

Since the therapeutic dosage of boron is about the same as the amount you can get from food, it is probably fairly safe. Unpleasant side effects, including nausea and vomiting, are only reported at about 50 times the highest recommended dose.

One potential concern with boron regards its effect on hormones. In at least two small studies, boron was found to increase the body's own estrogen levels, especially in women on estrogen-replacement therapy.
1 Because elevated estrogen increases the risk of breast and uterine cancer in women past menopause, this may be a matter of concern for those who wish to take supplemental boron. Further research is necessary to discover whether boron's apparent effect on estrogen is a real problem or not. At the present time, we would recommend getting your boron from fruits and vegetables: a large study found that high intake of boron from these sources did not affect breast cancer rates.
2

Interactions You Should Know About

If you are taking:

Hormone-replacement therapy : Use of boron may not be advisable due to the risk of elevating estrogen levels excessively.